The present invention relates to screen printing and particularly to screen printing in which pasty product to be printed is contained within a screen printing head and delivered through the printing head by applied pressure.
It is an established technique in the assembly of printed circuit boards to deposit solder paste where connections are to be made with components, place the components on the paste deposits, and then heat the assembly to re-flow the paste and complete the connections. Screen printing machines have been used to deposit solder paste onto printed circuit boards through the apertures of a stencil or screen.
Solder paste consists of metallic microspheres of solder joined by an organic material or flux. The metallic content of such solder paste typically makes up 50% of the volume, and up to 90% of the weight of the paste. The viscous flux consists of rheologic agents, adhesive agents and cleaning agents some of which are thixotropic and others of which are volatile solvents. The thixotropic property of the solder paste has the effect that relative movement of regions within the paste causes a process of shear thinning to locally reduce the viscosity of the paste.
A typical print will comprise a multiplicity of small blocks of solder paste, and for consistent quality it is essential that each block contains the same proportions of each constituent material. This requires a consistent homogeneous distribution of the materials within the solder paste.
In one traditional screen printing technique, as illustrated in FIG. 1, an inclined squeegee 1 is used to push a volume of a pasty product 2 over a stencil 3 which includes apertures 4 and is located above a circuit board 5, thereby filling the apertures 4 in the stencil 3 and providing a deposit on the circuit board 5. Forward movement of the squeegee with a horizontal force F1, causes a downward Force F2 to be applied to the pasty product 2. This downward force F2 forces the pasty product 2 into the apertures 4 in the stencil 3, and in conjunction with the adhesion of the pasty product 2 to the stencil 3 causes the pasty product 2 to roll across the stencil 3 as depicted by arrow 6, thereby shear thinning the pasty product 2.
There are many problems associated with this screen printing technique. One problem is that exposure of the pasty product 2 to the atmosphere results in evaporation of the solvents of the pasty product 2 and hence drying of the pasty product 2. Another problem is that increasing the speed of the squeegee 1 to increase the downward force F2 which forces the pasty product 2 into the apertures 4 of the stencil 3, not only reduces the time available to fill the apertures 4, but can also cause the pasty product 2 to slide across the stencil 3, thereby reducing the rolling effect and hence the shear thinning.
Screen printing heads have been proposed, for example as disclosed in U.S. Pat. No. 4,622,239, which enclose the pasty product to overcome the problems of evaporation, but these printing heads have not addressed the problem of setting a suitable print speed.
WO-A-96/20088 discloses a screen printing head which attempts to overcome both the problems of evaporation and the setting of the print speed by applying a pressure directly to the pasty product. However, this printing head does not provide for a rolling action of the pasty product and hence shear thinning of the same. Furthermore, this printing head requires a very high pressure to be applied to the pasty product. This high pressure can result in the separation of the metallic and flux components of solder pastes which results in inconsistent printing.
WO-A-98/16387 discloses a screen printing head which has been developed partially in response to the known problems of evaporation and the setting of the print speed. As illustrated in FIG. 2, this printing head comprises a main body 10, first and second wiper blades 11, 12, which contact a stencil 13 and together with the main body 10 define a chamber 15 containing a pasty product 16, a grille 17 located at the lower end of the main body 10, and a piston 18 for applying a downward force F2 on the pasty product 16. The stencil 13, which includes a plurality of apertures 19, is located above a circuit board 20 onto which deposits of the pasty product 16 are to be printed. In use, the printing head is moved in one of two opposite printing directions, with a horizontal force F1, which causes the wiper blades 11, 12, which are pressed against the stencil 13 by the force imparted on the pasty product 16 by the piston 18, to act to lift the pasty product 16 from the region above the stencil 13 and cause the pasty product 16 to pass upwardly through the grille 17, which pasty product 16 is subsequently forced back downwardly through the grille 17 by the action of the pressure developed by the piston 18. This rolling action of the pasty product 16, as depicted by arrows 21, shear thins the pasty product 16 and thereby enables the pressure F2 applied by the piston 18 to be maintained at a low level and also prevents separation of the components of the pasty product 16 by the mixing effect of the rolling action. Further, the pressure F2 applied to the pasty product 16 is independent of the speed of movement of the printing head.
Whilst this printing head provides for much improved screen printing, it has been established that in some circumstances this printing head does not provide for sufficient shear thinning of the pasty product 16 as necessary for a perfect print.
In the screen printing process, as illustrated in FIG. 3a, incomplete tilling of the stencil apertures 19 can result where the pasty product 16 is not sufficiently thinned. Where the stencil apertures 19 are incompletely filled, the action of the trailing wiper blade 11 can shear the pasty product 16 over the stencil apertures 19, with the result that the pasty product 16 in the apertures 19 is pushed to one, the forward, end of the apertures 19, as illustrated in FIG. 3b, resulting in only a partial print. In an extreme case, the remaining pasty product 16 in the stencil apertures 19 may not be in sufficient contact with the circuit board 20 such that when the circuit board 20 and the stencil 13 are separated, the pasty product 16 is insufficiently adhered to the circuit board 20 and remains in the stencil 13, as shown in FIG. 3c, resulting in virtually no print at all. Furthermore, the retention of pasty product 16 in apertures 19 of the stencil 13 can itself lead to problems with subsequent prints since, as mentioned hereinabove, the pasty product 16 is prone to drying out and the drying out of pasty product 16 in the apertures 19 will give rise to printing problems. These problems have been made worse by recent trends in miniaturization which have led to the use of smaller stencil apertures 19, which miniaturization reduces the area of pasty product to circuit board contact relative to the area of pasty product to aperture wall contact.
Accordingly, the present invention provides a screen printing head for applying a pasty product to a printing screen, comprising: a main body; wiper blades disposed to the main body for contacting a printing screen; a first chamber providing a reservoir for containing a supply of pasty product, the first chamber being defined at least in part by the main body and including at least one outlet opening through which pasty product is in use forced under pressure; a second chamber in fluid communication with the at least one outlet opening, the second chamber being defined in part by the main body and the wiper blades and being in use in communication with the printing screen; and a flow director disposed in the second chamber and configured such as in use to cause a circulatory flow of pasty product contained therein which passes over the surface of the printing screen and a flow of pasty product towards the printing screen which acts to force pasty product of the circulatory flow towards the printing screen and into apertures therein.
Such a printing head provides for enhanced shear thinning of the pasty product filled into the apertures of the printing screen and hence improved stencil aperture filling to reduce the incidence of poor quality printing.
Preferably, the at least one outlet opening comprises an elongate slot.
Preferably, the flow director is further configured such as to define first and second circulation zones in which pasty product is locally circulated and through which the circulatory flow is directed.
Preferably, the flow director comprises vanes, with the circulatory flow in use passing beneath lower edges of the vanes.
More preferably, the flow director comprises first and second vanes disposed on opposed sides of the at least one outlet opening with the lower edges of the vanes defining a nozzle directed towards the printing screen.
Preferably, the nozzle is in elongate nozzle.
Preferably, the main body includes first and second lobe members which in part define the second chamber, the lobe members being disposed above respective ones of the wiper blades and having arcuate lower surfaces to promote the circulatory flow.
In one embodiment the main body includes one or more ports through which the first chamber can be charged with pasty product.
In another embodiment the main body includes a replaceable cassette which defines at least in part the first chamber, the first chamber being charged by replacement of the cassette.
Preferably, the wiper blades comprise flexible wiper blades.
The present invention also extends to a screen printing apparatus comprising the above-described printing head.
The present invention also provides a method of screen printing using a screen printing head including a chamber in communication with the printing screen, comprising the steps of: providing a circulatory flow of pasty product in the chamber which passes over the surface of the printing screen; and providing a flow of pasty product towards the printing screen which acts to force pasty product of the circulatory flow onto the printing screen and into apertures therein.
Preferably, the method further comprises the step of locally circulating pasty product in first and second circulation zones through which the circulatory flow is directed.